Vortex flow measuring transducers are used for exact measurements of flow velocity or volume flow of a fluid flowing in a flow direction. German Patent, DE 10 2009 001 526 A1 describes such a measuring transducer having a measuring tube, wherein the measuring tube contains a bluff body, which is upstream of a vortex sensor. The bluff body serves for producing a Kármán vortex street in the fluid. Pressure fluctuations thereof are transformed by a downstream vortex sensor into an electrical signal, wherein the frequency of the measured vortices is proportional to the volume flow.
Vortex flow measuring sensors (or also vortex sensors for short) for such measuring transducers are known from the state of the art, among others, from European Patent, EP 0 841 545 B1. Described therein is a capacitive vortex sensor, which is inserted into a measuring tube and used there for measuring flow velocity or also volume flow. Essentially, the vortex sensor comprises a housing, within which is arranged a membrane, on whose side, which is facing the fluid flowing in the measuring tube, a sensor paddle is secured. Arranged on the side of the membrane facing away from the fluid is a capacitive electrode arrangement. One electrode is, in such case, connected with the membrane. The sensor paddle is caused to oscillate and is deflected by the vortices of the Kármán vortex street. This deflection transfers proportionally to the electrode arrangement, whereby a capacitance change occurs, which can be sensed.
Vortex sensors have, as a rule, a limited pressure range, in which measurement is possible. The upper limit of the pressure range is determined by allowable maximum values of membrane stresses, respectively bending stresses, which act on membrane and paddle. In order to expand the pressure range to higher pressure values, high strength materials are applied, which can withstand higher bending stresses. However, these materials, such as, for example, high strength steels, are expensive and complex to process.
Known from the state of the art of pressure sensors is an overload protection for pressure difference sensors equipped with membranes. Thus, an overload protection system for pressure sensors is described in “Overload-Resistant Pressure Sensor In the Nominal Range of 10 mbar(1 kPa)”-T. Kober, R. Werthschützky, Institute of Electromechanical Design, Technische Universität (Technical University), Darmstadt, Germany, Proc. Eurosensors XXIV, Sep. 5-8, 2010, Linz, Austria as well as in “Mikromechanischer Überlastschutz für Drucksensoren durch strukturierte Gegenlager aus Glas (Micromechanical Overload Protection for Pressure Sensors Using Structured Counterbearings of Glass)”-T. Kober, R. Werthschützky, Sensoren and Messsysteme (Sensors and Measuring Systems) 2010, Berlin: VDE Verlag GmbH (publisher), ISBN 978-3-8007-3260-9. Described therein is a micromechanical overload protection system produced from thermally treated glass for use in pressure difference sensors. Introduced into a silicon wafer by laser machining are defined cavities. Then, a glass wafer and the prepared silicon wafer are positioned next to one another. Thermal treatment causes the glass to form into the cavities. Then a silicon measuring plate is positioned, which serves as the pressure membrane. In the case of an overpressure loading from above, the silicon measuring plate moves from its resting position downwards until it touches the prepared glass wafer, which protects it from breaking.
Disadvantageous in the aforementioned state of the art, however, is that in the case of overloading of the silicon membrane, thus after a certain pressure, the membrane lies completely against the overload protection. While the membrane can thereby be effectively protected against the overload and, thus, from damage, still, the membrane can then no longer register oscillations. The measuring sensor formed with the membrane can no longer produce measurement signals.